Measuring 3 Phase Current: Tips for Determining Motor Draw

[chronic_mka]

Hi, I have a question.
Suppose I have a motor that runs on 3 phase power. If I only wanted to find out the total amount of current that the motor draws, how would I go about doing that?

Would I just use a clamp meter to get the amount of current going through each phase and then add them up to get the total current that the motor draws?

 

[MATLABdude]

Welcome to PhysicsForums!

Unfortunately, I'm not a power engineer and the last time I encountered these sorts of problems were in second year, nearly a decade ago. That said, you'll notice that the voltage (and hence current) waveforms are all 120 degrees out of phase:
http://en.wikipedia.org/wiki/3_phase

Your clamp meter (usually) gives you an RMS value for current (though you can also get ones that give you the max and min), or, the equivalent DC current that would be have to flow to give the same power. With this in mind, you can add up the RMS currents per phase to determine total current consumption (what the power company would charge you).

EDIT: Hopefully, your currents per phase are fairly close, otherwise your motor (or the load) may be unbalanced!

 

[zgozvrm]

If everything is balanced, all three legs should show the same amount of current (albeit, at different angles). In practice, this is not the case, so I usually just consider the highest current reading.

 

[MThornton]

It is not uncommon for typical 3-phase supply voltages to be slightly unbalanced. this voltage unbalance will be reflected in per-phase current imbalance in the motor. It is acceptable & normal to see a few percent of per-phase current imbalance. Just take the average of the 3.

Power for a 3-ph motor = V(line-line) x I(per-phase avg) x 1.732(root of 3) x PF
PF is power-factor, which is the cosine of the phase angle between the applied voltage and the phase currents. For a small AC 3-ph motor, this will be between 0.75 and 0.90, depending on the load. The motor's full-load PF should be printed on the name-plate.

M

 

[zgozvrm]

It is not uncommon for typical 3-phase supply voltages to be slightly unbalanced. this voltage unbalance will be reflected in per-phase current imbalance in the motor. It is acceptable & normal to see a few percent of per-phase current imbalance. Just take the average of the 3.

Taking the average is okay, but I generally like to consider the highest reading when sizing components (wire, contactor, fuses, etc.). As things get older and worn (contact resistance, connection torque, insulation breakdown, etc.), they are likely to cause higher current in the motor, so I'd rather oversize a bit anyway. In most cases the average reading and the highest reading will yield the same results as far as component sizing, but when those results are "on the line" between one size and the next, I'd use "the next."

Note that more than a few percent (>5% roughly) of imbalance generally is unacceptable, and indicates a possible problem (especially in a new motor). Sometimes this can be due to an unbalanced voltage and rotating the motor wires may "even things out."